Add query string parameter to search by recipient

> Service teams that use the admin interface often need to know the
> outcome of a message... at the moment they have to page through all
> the results in the activity stream. They should be able to find
> notifications by email address or phone number.

– https://www.pivotaltracker.com/n/projects/1443052

This commit adds an additional query string parameter (`to`) to the URL,
which users can use to filter down the list of notifications.

It:
- takes the status into account
- doesn’t update the counts based on the search term (in reality each
  service will only send a handful of notifications to one person in any
  7 day period)

In other words the funnel that filters down the notifications looks
like:

> all notifications for service → only failed → only to this phone
> number

app/templates/views/notifications.html

@@ -20,7 +20,7 @@
 
   {{ ajax_block(
     partials,
-    url_for('.get_notifications_as_json', service_id=current_service.id, message_type=message_type, status=status, page=page),
+    url_for('.get_notifications_as_json', service_id=current_service.id, message_type=message_type, status=status, page=page, to=to),
     'notifications'
   ) }}